The evolution of cell adhesion and signaling mechanisms was essential for the origin of animal multicellularity. These innovations facilitated multicellular development and cell differentiation, and helped resolve the evolutionary conflict between cooperation and competition in single celled organisms. Modern animal development and physiology (including in humans) therefore rests upon a regulatory foundation nucleated by ancient adhesion and signaling mechanisms. Cancer biology is perhaps the most prominent example of how ancient tensions between cell cooperation and competition can re-emerge when fundamental regulatory mechanisms are compromised. By studying choanoflagellates, the closest living single-celled and colony-forming relatives of animals, we aim to determine the minimal molecular toolkit for animal multicellularity. Whole genome and EST sequences from two choanoflagellates, Monosiga brevicollis and Proterospongia, reveal that choanoflagellates express homologs of genes required for cell signaling and adhesion in animals (e.g. receptor tyrosine kinases and cadherins). In addition, Proterospongia develops highly organized colonies in which neighboring cells are connected by cytoplasmic bridges that we hypothesize mediate intercellular signaling and adhesion. Using choanoflagellate-specific molecular, genomic, and cell biological resources generated by my lab, we study choanoflagellate colony formation as a simple model for the evolution and development of animal multicellularity. We propose three aims to determine the mechanisms of intercellular interactions in Proterospongia colonies. First, we will characterize the development and ultrastructure of cytoplasmic bridges and measure their ability to act as intercellular conduits for signaling molecules. Second, we will use proteomic and functional genomic approaches to identify molecules associated with the formation of cytoplasmic bridges. Third, we will investigate the localization and function of choanoflagellate homologs of animal signaling, adhesion, and midbody proteins. By uniting cell biology, functional genomic and biochemical approaches in the study of intercellular interactions in Proterospongia, this research will offer new insights into the molecular underpinnings of animal development and cancer.